The CO2 gas Cherenkov detectors for the Jefferson Lab Hall-A spectrometers

Two threshold gas Cherenkov counters have been constructed for the electron and hadron High Resolution Spectrometers (HRS) of the Jefferson Lab Experimental Hall-A. These counters are intended to separate electrons/positrons from other particles up to 4 GeVc. The counters are operated at atmospheric pressure with CO2. Each counter is equipped with ten mirrors. Light weight, thin spherical mirrors (∼ 5.5 × 10−3 radiation lengths) have been employed resulting in a total thickness of ∼ 1.4 × 10−2 radiation lengths crossed by the particles. A prototype of the counter has been tested at CERN with a mixed beam of positrons, pions and protons from 1 to 4 GeVc. Its detection efficiency for positrons and the rejection ratios for pions and protons have been measured as a function of the pulse height response (or equivalently the number of photoelectrons). An improvement of 34% in the number of photoelectrons has been obtained by using a wavelength shifter coated on the photocathode glass window. With such an improvement in 1 m long radiator, an inefficiency for positrons less than 10−3 and rejection ratios πe at the level of few 10−3 and pe smaller than 10−3 have been obtained for pulse heights above 2 photoelectrons. Contaminations of particles below the Cherenkov threshold is fully understood considering δ-rays production.